– “Closing off specified areas and not letting people out”
That is the correct way to do it; remember what happened in December. The government announced in advance that London would be subjected to Tier 4 restrictions on a particular date, which provoked a mass exodus from the most infected part of the country to all other regions, spreading and accelerating the outbreak and thereby prolonging the eventual lockdown.
I would much rather that local people’s assemblies would enforce their own restrictions, allowing people in only from other Green Zones. This would be far more robust; everyone within a locality would have a very strong incentive to protect their infection-free status, so people would cooperate and back each other up. This is localism and anarchism’s approach to epidemic control. Alas, nearly the entire population have displaced their agency and responsibility onto the utterly corrupt national government, which fails them repeatedly and comprehensively.